The present invention relates of a method of operating a heat storage system in a more efficient manner by altering the thermal gradient within the heat storage unit.
Heat storage units particularly adapted for use in the present invention are those of the heat exchange type which contain a bed of heat storage medium. The heat storage medium may suitably be a heat of fusion type which cycles from solid to liquid and vice versa. Particularly useful are alkali metal hydroxide compositions. Such compositions are particularly well suited because of their high heat storage capacities, high heats of fusion, broad operative ranges, relative inertness, and their low vapor pressures.
Alkli metal hydroxides have melting points ranging from about 272.degree.C. for cesium to about 450.degree. C. for lithium. The incorporation of additives such as corrosion inhibitors and non-reducing agents into the alkali metal hydroxide heat storage compositions facilitates the production of useful mixtures with a variety of melting points. Sodium hydroxide, the most commonly available of the alkali metal hydroxides, is aptly suited to use in compositions useful as the heat storage medium in the present invention. Sodium hydroxide compositions of relatively high purity have a melting point of about 318.degree. C. Such compositions may consist of liquid - solid mixtures in the range of from about 232.degree. to about 340.degree. C. During a heat storage cycle, sodium hydroxide compositions may be heated to temperatures as high as 675.degree. C. without harm. Normal operating temperatures of heat storage units containing sodium hydroxide compositions as a heat storage medium range from about 100.degree. to about 500.degree. C.
In use, the heat storage medium is placed in a container, such as a tank, to form a bed. The container, preferably elongated, is equipped with a plurality of pipes or tubes to facilitate good heat exchange within the container. A heating fluid, suitably water, at a relatively high temperature, is passed longitudinally through the bed of heat storage medium. The heating fluid adds heat to the heat storage medium and exits the container at a temperature lower than that at which it entered. The fluid to be heated, suitably also water, is passed longitudinally through the bed counter current to the flow of the heating fluid. The fluid to be heated enters the bed at a relatively low temperature and exits at a higher temperature. In operation of a heat storage - heat exchange unit, a temperature gradient, a temperature difference, is established within the bed. A relatively cool, or colder zone is formed nearest the incoming fluid to be heated, and a relatively hot zone is formed in the area of the entering heating fluid. As the unit is operated, a temperature gradient is established and tends to spread through the medium bed. Thus, the gradient, or temperature difference, becomes incrementally graduated throughout the bed.
The colder end of the heat storage unit may be viewed as heat storage capacity that is available and unused. In a preferred mode, the present invention facilitates the use of this heat storage capacity.